Why Does California's Central Valley Have Such Bad Air Pollution?

As we reported earlier this week, the World Health Organization released its latest report measuring air quality in cities all over the world. The report specifically looks at the concentration of particles measuring 10 micrometers or less – those likely to get into the blood stream and cause disease.

Of the 375 U.S. cities included in the list, only 36 of them exceed the WHO’s air quality standard of 20 micrograms of particulates per cubic meter, on average. That’s pretty good. But of the ten worst performing cities, five are located in California’s Central Valley.

So what’s going on here?

The Central Valley of California, anchored by cities like Fresno, Bakersfield, and Modesto, is the farming center of the state, and really the country. Its eight counties grow about half of the nation’s fruits and vegetables. This fact is largely due to its geography – a wide valley that’s also the unfortunate cause underlying much of its persistent problems with air pollution.

Surrounded on three sides by mountain ranges, the Central Valley acts as a pool for pollutants produced by the region’s roughly 3.5 million residents, its industry and its large agricultural community. These emissions get trapped in the valley by an inversion layer of warm air, explains Dimitry Stanich of the California Air Resources Board.

“California has had the worst smog problems in the States for 40 years,” Stanich says.

And the Central Valley is feeling the brunt of it. In addition to the Valley’s unique geographic and meteorological conditions, it’s also a growing population center. Fresno County, the most populous in the region, grew by 16 percent between 2000 and 2010, and now houses more than 930,000 people. More people, and more cars.

“One of the big things we’re dealing with is that we have a 1 to 2 ratio of people to vehicle miles traveled,” says Jaime Holt at the San Joaquin Valley Air Pollution Control District.

These mobile sources of emissions add to the Valley’s problems, but Holt argues they’re not the main cause. The region’s agriculture is responsible for much of the region’s pollution. Up until a few years ago, farmers in the region would regularly burn brush and cuttings at the end of the season, creating huge sources of particulate matter in the air. A new state law, enforced since 2004, regulates the emissions of the agriculture industry in the state, and Holt says the Valley’s pollution problems have already started to decline. In 2002, more than 4,600 tons of 2.5-microgram particulate matter was recorded. In 2008, that figure was down to 1,600 tons.

The problem is getting better, but it’s by no means solved. As agricultural burn-offs continue to decrease, the Valley can expect to see its air quality improve. But regardless of the value of those improvements, its geography and meteorology distinctly disadvantage it to suffer below average air quality.